It is desirable that an elevator button have a number of characteristics not required in other buttons. Such a button must be able to withstand a greater impact force because the average passenger often does not push an elevator button, but rather he/she strikes it. A vandal may strike it even harder. Second, elevator buttons must be resistant to heat so that they are not damaged in a fire nor register calls in a fire. They must be similarly unresponsive to flame. Finally, it is desirable, both for aesthetic reasons and functional reasons that the button not have perceptible motion to register a call for service. An elevator button with no moving mechanical parts will be less likely to be subject to the need for repair.
Three are two types of piezoelements commonly used for pushbuttons: piezoresistive and piezoelectric.
Buttons using a piezoelectric element are based on the piezoelectric effect, i.e., the material in the button generates a voltage potential when subjected to pressure. Such a button is disclosed in U.S. Pat. No. 4,805,739, "Elevator Control Switch and Position Indicator Assembly", by Lind et al, issued Feb. 21, 1989. Buttons incorporating piezoresistive elements use the fact that the resistivity of the piezoelement changes with pressure.
One problem with piezoelectric materials is that they are brittle, a characteristic wholly unsuited to elevator buttons because of the daily rigors to which they are subjected. Such buttons are defenseless against vandals. The brittleness of piezoelectric materials lends them to being easily shattered.
A second problem with piezoelectric buttons is that they produce an output signal that is difficult to process with digital circuitry. The output of the piezoelectric element, when subjected to a flat load curve such as a step function, is a signal similar to the output from a high past filter where the time constant is much smaller than the time period of an input step function. Third, piezoelectrics are particularly sensitive to damage in a fire due to their heat sensitivity.
Attempts have been made to produce an elevator button with the desirable qualities mentioned above. Such an attempt is found in "Pathless Piezobuttons For Industrial Applications", components report 14 (1975) No. 5, which displays a piezoceramic film utilizing the piezoelectric effect. In the reference, the travel involved by the finger pressure is less than 1 micrometer. Hence, the name "pathless". The piezoelectric material in this button does not have the inherent temperature resistant characteristics needed for an elevator button. It therefore requires a flame-protective material for the button housing in order to overcome this deficiency. The reference discloses a piezoelectric member located perpendicular to the button face and a button backing having support members which secure the element vertically.
Other buttons similarly are unable to meet the needs of elevator systems. U.S. Pat. No. 4,267,725 discloses a vertical piezoresistive element braced at its center by a set screw such that depression of the button face at the upper tip of the element, and transverse to it, causes the element to generate a signal proportional to bending of the element. The set screw serves to ensure that the bending is only in one direction. This button fails to fulfill the need for a button with no observable mechanical movement since the button face of the reference must descend in order for the piezoelement to bend and thus give a signal. The nature of thin film silicon is such that a large amount of deformation is required to change the resistance of the element significantly enough that it can be measured. Such deformation is achieved, with the least amount of force, by bowing the element; it could not be so easily achieved by applying pressure transverse to the element on a flat surface. The reference requires a large number of complexly interrelated moving parts because there is no allowance to choose not to bow. Accordingly, this design is undesirable.
In another button, disclosed in U.S. Pat. No. 4,644,315, a cantilevered piezoresistive element is used. In one embodiment the cantilevered element is vertical while in another the cantilevered element is horizontal. This reference discloses a donut-shaped magnet connected to a plunger on a button face such that the magnet descends with the button face to deflect an oppositely-poled magnet mounted on the tip of a cantilever having a piezoresistor hinge. This button, however, does not meet the need for an elevator button having no mechanical movement. Further, the use of the magnet/hinge apparatus and the need to position the magnet/hinge assembly with great precision make for a button that is costly. The same factors make it likely to be difficult to manufacture insofar as setting the magnet/hinge arrangement such that it interferes with the donut-shaped magnet so much as to sense its movement, but not so much as to be directly in the path of the donut-shaped magnet to as it descent. Also, this reference does not achieve a signal output readily susceptible of processing through digital circuitry. The second embodiment shows the cantilever mounted in horizontal fashion. This tends to produce more equal forces of attraction and repulsion leading to more symmetrical output pulse shapes. However, again, the fine tuning of the positioning of the hinge/magnet arrangement as well as the need for moving parts make this an undesirable button.
Finally, U.S. Pat. No. 2,632,062, "Semiconductor Transducer", is predicated upon the discovery that a PN junction, when subjected to pressure, changes resistance. The patent shows in several embodiments, various elements, shaped like pencil tips, pressing upon a piezoresistive PN junction. It also shows one or more axles, in other embodiments, passing through one or more PN junctions. Pressure upon the axles subjects the PN junction to pressure, yielding a change in resistance. This reference does not satisfy the needs of elevator button. The silicon material is not resilient. The pencil tip used to stress the PN junction would not lead to a button with a long life, while the axle embodiment is quite inappropriate for use in an elevator--it does not disclose a button.